Photoneutron Yields in the Rare-Earth Region

Abstract

The total photoneutron yield curves for Sn, I, La, Ce, Sm, Tb, Ho, Er, Yb, Ta, Au, and Pb have been measured for x-ray energies from 7 to 40 Mev with about one percent statistical uncertainty. The Penfold-Leiss matrix was used to convert these yield curves to integrated neutron yield cross sections directly without smoothing the original activation curves. The cross sections derived from the integral curves were corrected for multiple neutron emission above the ($\gamma , 2n$) threshold using the statistical model. The widths found for the giant resonances for the closed-shell nuclei decreased from 5 Mev to 3.8 Mev in going from Sn to Pb. The widths for the elements having large nuclear deformations for most of their isotopes were considerably broader. These widths decreased slowly from 8.6 Mev for Sm to 6 Mev for Ta. These widths are consistent with the broadening of the giant resonance to be expected on the Danos model if values of the intrinsic quadrupole moment are taken from Coulomb excitation data. The neutron yield cross sections corrected for multiple neutron emission were integrated to 22 Mev. Using these integrals and defining $f$ by $\int \sigma \mathrm{dE}=0.06\left(\frac{\mathrm{NZ}}{A}\right)f$, these data gave an average value of $f=1.34\mathrm{}\pm 0.21$.